#include<iostream>
using namespace std;
int main()
{
int L,I;
cin>>I;
if(100>=I>=90)
L=1;
else
if(90>I>=80)
L=2;
else
if(80>I>=70)
L=3;
else
if(70>I>=60)
L=4;
else
if(60>I>=0)
L=5;
else
cout<<"1748"<<endl;
switch(L)
{
case 1:
cout<<"A"<<endl;
break;
case 2:
cout<<"B"<<endl;
break;
case 3:
cout<<"C"<<endl;
break;
case 4:
cout<<"D"<<endl;
break;
case 5:
cout<<"E"<<endl;
break;
default:
cout<<"1314"<<endl;
}
return 0;
}

100>=I>=90,这种写法有问题。
改成100>=I && I>=90.

#include<iostream>
using namespace std;
int main()
{
int L,I;
cin>>I;
if(I<100&&I>=90)
L=1;
else
if(I<90&&I>=80)
L=2;
else
if(I<80&&I>=70)
L=3;
else
if(I<700&&I>=60)
L=4;
else
if(I<60&&I>=0)
L=5;
else
cout<<"1748"<<endl;
switch(L)
{
case 1:
cout<<"A"<<endl;
break;
case 2:
cout<<"B"<<endl;
break;
case 3:
cout<<"C"<<endl;
break;
case 4:
cout<<"D"<<endl;
break;
case 5:
cout<<"E"<<endl;
break;
default:
cout<<"1314"<<endl;
}
return 0;
}

LZ，用大括号标上吧
不要省力
也不至于要神秘

改了，行了，谢谢楼的的朋友们。。。。